This invention pertains to an improvement in the method and apparatus for the controlled partial decomposition of a solid carbonaceous material into combustible gases and an oxidizable solid residue.
More particularly, my invention relates to the calcining of petroleum coke.
Traditionally, such calcining has been done in a rotary kiln. The rotary kiln is a long, rotating tube, generally called a shell, lined with a refractory and mounted at a slight angle from the horizontal. Uncalcined coke, called green coke or raw coke, is fed into the kiln at the upper end, and heat is supplied at the lower end or firing crown by the burning of a heating fuel of combustible liquids or gases; for example, fuel oil, bunker oil, natural gas, synthesis gas, producer's gas, or the like. Coke passes through the kiln toward the firing crown by tumbling through the rotating shell. Substances that are volatile at the high temperatures generated in the kiln are evaporated and are partially burned by secondary air fed into the kiln around the firing crown. In the usual kiln design, all of the excess air necessary to provide the oxidative atmosphere to consume some or all of the evaporated combustible material from the coke is provided at the firing crown and passes the length of the kiln.
In the description, air is conveniently called primary air when it is mixed and fed with the heating fuel, and secondary air when it enters the kiln at and around the rotary seal at the firing crown end of the kiln.
Several modifications of this design have been disclosed in the prior art. Among the prior art methods controlling air flow in the kiln are those of Walden, U.S. Pat. No. 1,564,730; Nielsen, U.S. Pat. No. 1,908,651; Borch, U.S. Pat. No. 2,710,280; and Collier, U.S. Pat. No. 2,813,822.
In the Walden modification, the kiln is equipped with tuyeres arranged symmetrically about the shell and disposed along a considerable portion of the length of the shell. The tuyeres are equipped with regulating devices to allow for control of the air being admitted to the kiln, and nozzles or air outlets on the inside of the kiln. The nozzles direct the air within the kiln either radially or axial to the longitudinal axis of the kiln with nozzle openings facing both the feed end and firing crown.
Nielsen provides that both the combustible gas inlet, at the firing crown, and the volatile product exhaust, at the feed end, be modified to consist of pipes fitted with or expanded to include conical extensions within the kiln for the purpose of providing and controlling a swirling movement to the heated gases in the kiln.
Borch provides an electrically heated kiln wherein, during the passing of coke through a preheating zone, both the sensible and latent heat of the expelled volatiles are utilized by burning the volatiles by means of air supplied by pipes (tuyeres) through the walls of the kiln. The air from these pipes is directed toward the discharge end of the kiln in a stream parallel with the longitudinal axis thereof. The area of burning is isolated, so as to not include the heating zone, by control of the air intake and volatile product exhaust.
In all of the preceding modifications, the oxidizing gases are admitted to the kiln at or near the central longitudinal axis of the kiln, or at least above the coke bed. Collier discloses a traditional countercurrent rotary gas fired kiln wherein additional air is admitted in the preheating zone but at the surface of the refractory lining of the kiln below the coke bed with the nozzles disposed in the refractory material. Collier does not provide for a volatile product exhaust in the preheating zone as does Borch, and the combustible products are discharged along with the rest of the gaseous exhaust products at the cokefeed end.